(1) The Mg(II)-ATP-dependent phosphoprotein phosphatase in skeletal muscle is inhibited by a cAMP-dependent protein kinase (type II). This inhibition is due to the binding of the dissociated regulatory subunit to the phosphatase. Although the catalytic subunit of the cAMP-dependent protein kinase is capable of phosphorylating the modulator subunit of the phosphatase, unlike the kinase Fa it fails to activate the inactive phosphatase. (2) In an attempt to identify endogenous brain substrates for the Ca(II)-calmodulin-dependent protein phosphatase, the catalytic efficiencies (kcat/Km) for several well characterized phosphoproteins purified from brain were evaluated. The results indicated that DARPP-32, G-substrate and protein K.-F. are potential substrates for the phosphatase. (3) The phosphorylation and dephosphorylation of smooth muscle myosin involves cooperative interaction between two heads when myosin is polymerized into filaments; but with monomeric myosin both phosphorylation and dephosphorylation occur randomly.